Ultra high frequency signal generator



Dec. 26, 1950 A v HAEFF r 2,535,062

ULTRA HIGH FREQUENCY SIGNAL GENERATOR 4 Sheets-Sheet 1 Filed April 28, 1945 6.2: Pr whim IIIII llllll lll ll IL rlllll. I'll-Ill III Andrew V. Huff WM mm MB cm mm Tc 4 Sheets-Sheet 2 .0 an? to W 23 o: 5.93: 8 0 "GDP to VWM EN 0. do do 82F uo 05.2w: 2. A

Dec. 26, 1950 A. v. HAEFF ETAL ULTRA HIGH FREQUENCY sxcmu. GENERATOR Filed April 28, 1945 I I l I I I l I I l l I l l I I I I I I l I I I I l l I I I l I I l l I l l I l l I I I I I I I l l I I l l I l I I I I I I 2 AndI-cwV. Ht! Thur: E. Hamleq Chm-Ina ash-nth Dec. 26, 1950 A. v. HAEFF ETAL ULTRA HIGH FREQUENCY SIGNAL GENERATOR 4 Sheets-Sheet 3 Filed April 28, 1945 mmnunbhbu wmuhcwzjt 9-.

Wm M mhm v mum mmm ATC W0 zuhcmz 6 F. nm unara.

Dec. 26, 1950 A. v. HAEFF ETAL ULTRA HIGH FREQUENCY SIGNAL GENERATOR 4 Sheets-Sheet 4 Filed April 28, 1945 PULSE RATE VARIED BY R-309 PUlSE nah VARIED BY w Charles B. Smith (Luwu Patented Dec. 26, 1950 UNITED STATES PATENT OFFICE ULTRA HIGH FREQUENCY SIGNAL oansaa'roa Application April 28, 194;, Serial No. 890,881

(Granted under thea'ct or n a, 1883, as amended April 30, 1928: 70 O. G. 757) 14 Claims.

This invention relates broadly to improvements in signal generators and more particularly to an ultra high frequency signal generator adapted for unmodulated, modulated or pulsed operation.

Due to the wide application of the pulse technique in the ultra high frequency field, there has been a definite need for reliable ultra high frequency signal generators capable of producing short pulses of radio frequency energy of known amplitude and duration and tunable over a. wide range of frequencies in the microwave band. One important use of such a signal generator is, for example, in aligning, testing, and measuring the overall sensitivity and performance of ultra high frequency receivers and other high frequency electronic devices requiring a source of accurate, adjustable calibrated radio frequency voltage, unmodulated, modulated or pulsed. Many other uses of such a signal generator are readily recognizable. Attempts to adapt for pulse operation known types of commercially available standard signal generators have notbeen entirely successful since they have been found too unreliable and lacking in the desired output characteristics in the very high frequency ranges and above. 3

One of the objects of the present invention is to provide an improved signal generator for use at ultra high frequencies and higher frequency ranges.

Another object of the invention is to provide a signal generator capable of unmodulated, modulated or pulsed operation at ultra high frequencies.

A further object of the invention is to provide a simple and emcient signal generator for useat ultra high frequencies in the range substantially between 500 and 4000 megacyclea.

A still further obiect of the invention is the provision within an ultra high frequency signal generator' of a complete oscillation generation system capable of delivering a continuous radio frequency output which is either unmodulated, modulated or pulsed, of known amplitude and duration, in'

combinat.on with a wavemeter for insuring the adjustment of the oscillator frequency, an attenuator for accurately controlling the amplitude of the signal generator output, and a radio frequenoy shielding and filtering system for min- Another feature of the invention is the use of a special radio frequency iilter system in the signal generator circuit.

Other objects, features and advantages of the invention will be apparent from the following detailed description considered in connection with the accompanying drawings which disclose several embodiments of the invention. It is expressly understood however that the drawings are designed for purposes of illustration only and not as a definition of the limits of the invention, reference for the latter purpose being had to the appended claims.

In the drawings, wherein similar reference characters denote similar elements throughout the several views:

Fig. 1 is a. diagrammatic illustration partly in section of a signal generator constructed in accordance with one embodiment of the present invention;

Fig. 2 is a diagrammatic illustration of a signal generator of the present invention showing the pulser circuit and power supply thereof in greater detail;

Fig. 3 is a diagrammatic showing of a signal generator constructed in accordance with another embodiment of the present invention;

Fig. 4 is a plan view of the special radio frequency filter arrangement employed in the present invention;

Fig. 5 is a longitudinal sectional view through line 6-5 of Fig. 4, and v Fig. 6 is a, group of waveforms of various voltages developed upon operation of the Fig. 3 embodiment.

In its broad aspects the present invention provides signal generators composed of the following elements: A variable-frequency cavity oscillator, apulser, a power-supply. and a power-level monitor. The oscillator is of the grounded-grid type tunable over aband of frequencies having a pair of tunable circuits consisting of two concentric cylindrical resonant cavities,- one in the gridplate circuit and the other in the grid-cathode circuit, Means are included for feed-back coupling between the two cavities. A tunable attenuator is inductively coupled to the cathode cavity. Measurement of the power output of the oscillator is accomplished by the power-level monitor and determined by the setting of the attenuator and by the reading of an output meter in a balanced bridge circuit including a bolometer element associated with the power-level monitor system. A cavity-type wavemeter is associated with the oscillator and includes a rectifier, a tunable quarter wave coaxial line section loosely coupled to the plate cavity of the oscillator, and a reference meter in the rectified output of the wavemeter circuit for indicating resonance of the wavemeter with said oscillator. For pulsed operation of the oscillator an internal pulser system is provided comprising a vacuum tube asymmetrical multivibrator circuit, a pulser tube connected to the multivibrator circuit and keyed by the output thereof, an open-ended artificial transmission line in the cathode circuit of the pulser tube, and a keyer tube controlled by the output of said pulser tube. Means are provided whereby the cathode of the oscillator may be selectively connected to synchronizing or triggering signals supplied from either the output of-the internal pulser system or from pulses supplied from an independent, external pulse source. Means are included also for effecting either unmodulated or continuous wave operation of said oscillator. Radiation or leakage of the high frequency energy is minimized by the provision of a special filter system and radio frequency shielding and cathode tuning control knobs for separately indicating the position of the sliding plungers l8 and I 9 within the cavities. With such an arrangement, the revolutions of the frequency knob are counted on the revolution counter, appearing in tenths of a revolution from 0 to 100, and a frequency calibration chart is provided for each individual equipment which enables the desired frequency to be obtained by turning the frequency knob until the proper counter reading appears.

Oscillations are maintained by a fixed coupling lead 20 which provides feedback between the cavities I5 and ii.

The output of the oscillator i0 is taken from the plate grid cavity l6 by means of a stub 32 which operates as the primary of a mutual-inductance attenuator 30, which is to be described subsequently in greater detail, the secondary of which is the small loop 36 of the output attenuator system.

For an accurate adjustment of oscillator frequency a wavemeter 2| is provided which is built into the signal generator system as an integral whereby a calibrated'output voltage is obtained a from the signal generator throughout its entire frequency band. Ventilation is provided by an 'electric fan within the signal generator cabinet.

Referring now to Fig. 1, the radio frequency cavity oscillator In there shown is of the grounded grid type using a "lighthouse" tube H (type 446A triode) which comprises a disk-seal, parallel-plane design triode of the family now commonly referred to in the art by the generic 'term megatrons. The essential elements of the oscillator circuit are shown schematically in Fig. 1 wherein there is illustrated the relative arrangements of the various capacitances, induct- 'ances and other elements within the associated resonant cavities or tank circuits. For the purposes of discussion the elements of the lighthouse" oscillator tube I are illustrated schematically as consisting of an anode or plate l2, 9. cathare separated by a transverse partition I! in the plane of the grid H.

The radio frequency cathode l3 of the tube Ii is insulated inside the tube from the direct current cathode or heater I05 by a mica washer (not shown). The cathode I3 is provided with a filter including inductances HIS-I01 and capacitors i08-l09, and the heater I05 is supplied with a suitable voltage through a filter including inductances HG-l I! and capacitors H8 and H9. Plate 'voltage is supplied by way of lead H0 through a Ifilter comprising resistor H2 and capacitors i II and H4.

The cavities i5 and iii are independently tunable by means of the sliding spring contacts or plungers l8 and l9one in the plate-grid cavity and one in the grid-cathode cavity. The frequency of oscillation is determined by tuning the grid-plate and grid-cathode cavities, the frequency range being controlled by adjustment of the plungers I8 and [9 which are ganged together and slide in their respective cavities upon rotation of a lead screw (not shown) by means of separate frequency-control knobs (not shown) on the front panel of the signal generator cabinet. If desired, suitable counters (not shown) may be provided operatively connected to the plate part thereof. This wavemeter is designed to cover a frequency range from 500 me. to 4000 me. The wavemeter 2| consists of an accurately tunable quarterwave coaxial line section 22 which is loosely coupled to the plate-grid cavity It. The resonance of the wavemeter circuit may be indicated by a reference meter 25 suitably located on the front panel (not shown) of the signal generator. The meter 25 reads rectified current output of a crystal 28 coupled to the wavemeter circuit. The wavemeter frequency can be read from a wavemeter frequency calibration table and curve (not shown) for the particular resonant cavities i5 and I6 respectively, which dial scale and meter settings indicated. High Q -of the wavemeter circuit and a large reading fon the meter permit the setting of frequency with a relative accuracy of the order of 0.1 me.

The output attenuator system consists of a cylindrical waveguide type attenuator 30, of 0.5"

diameter tubing. The total range of attenuation is over db., so that a range of voltage output from a maximum of 0.15 volt to a small fraction of a microvolt is available. As illustrated, the attenuator input circuit consists of a coaxial line 3! having a stub 32 which is capacitatively coupled at the open end thereof to the oscillator circuit, and is shorted at the end 33 to which the attenuator is constantly monitored by means of -a bolometer wire 34 mounted near the shorted 'end of the coaxial line 31.

The output of the oscillator after attenuation is available by means of coaxial output cable 35 which consists of a "lossy cable coupled by means of a small loop 38 to the attenuator chamber 31, the loop 38 being connected to ground through a moving contactor inside the attenuator tube. The cable 35, which is a concentric semi-flexible cable approximately four feet in length, is attached to the attenuator by a suitable radio frequency coupling and brought outside of the signal generator cabinet through the center 'of the single calibrated attenuator control knob 38. Due to high attenuation of the cable 35 (over 10 db.) the effective internal impedance of the signal generator as it appears from the output terminal (cable connector 35) is substantially constant over the entire frequency range of the signal generator and is equal to the characteristic impedance of the output cable 35, which may be either 50 ohms or 70 ohms depending upon the 7 design of the signal generator.

The length of the "lossy" cable is usually determined by its attenuation at the lowest frequenoy which should be of the order of 6 to 10 db. In a signal generator covering very high frequency range this may result in loss in output voltage due to the excessive loss in the "lossy" cable at the high frequency end of the range. This is caused by increased attenuation of the cable at higher frequencies.

In order to obtain proper internal impedance of the signal generator over a wide frequency range without excessive loss in power output at high frequencies the "lossy" line may be used in combination with the usual terminating re sistors. Then. at low frequency the termination of the proper character is due to the presence of the resistor at the pick-up loop 36 of the output cable even though the attenuation of the loan cable is insufficient. At higher frequency, even though the terminating resistor has appreciable reactive impedance. the increased loss of the cable makes the effective impedance of the signal generator approach the character of pure resistance equal to the characteristic impedance of the output cable.

The attenuation is controlled by a known form of index nut and screw-threaded slide arrangement driven by gearing connected to the attenuator control knob 38. Turning the attenuator knob 38 moves the output coupling 100p 36 and varies the coupling within the attenuator chamber 31 and therefore the output voltage. The position of the attenuator is indicated by graduations on the attenuator knob 38 and by the counter 40. The threads per inch of the attenuator movement and the counter gearing are such that the counter 40 reads approximately in decibels below a specified level. The counter 40 has reading ranges from 0 to 100 db. in tenths of a decibel, changing 2 db. per revolution of the attenuator knob. The reading can be converted to output voltage by using a calibration chart provided with the equipment.

The attenuator can be set within 0.1 db. or better and it is so designed as to have straightline attenuation and is direct reading in decibels. Because of the loading which is caused by the attenuator stub it is impossible to track the plungers i8 and i9 over the range of frequencies covered by the signal generator. As will appear more fully hereinafter, a compensating impedance lcop may be connected between the plate and grid cylinders of the oscillator to make tracking possible. This loop loads the plategrid cavity and tracking can be accomplished only when the impedance of loop and the impedance of the attenuator stub 32 are equal over the whole frequency band.

In order to have a continuous indication of the oscillator power a precision bolometer-tape monitor device generally designated 42 is provided. It consists of a short length of Wollaston wire 34 (taken from 0.01 ampere Little-fuse) which is connected between the inner conductor or stub 32 of the attenuator input coaxial line II and ground through a suitable by-pass capacitor 46. The wire 34 forms one branch of a balanced Wheatstone bridge 43 connected to a suitable indicating meter 44 (microammeter) on the front panel of the device to indicate the degree of unbalance of the bridge. As above-mentioned, the bolometer element 34 is connected through capacitor 45 across the radio frequency oscillator circuit near the input to the attenuator system. The second (broken line) representation of resistor :4 shows its relative position in the bridge circuit 43. The other two branches of the bridge 43 contain resistors 44--48 each of equal resistive value. Resistors 41 and 48 are voltage dividers which provide the desired voltage drop across the bridge. A small D.-C. current supplied from a conventional type of regulated power supply passes through the bridge continuously. The bridge 43 is balanced for the condition of zero radio frequency power by means of an adjustable variable resistor 48 mounted on the front panel and used to adjust the meter 44 to zero before the oscillator is is turned on. when the oscillator is turned on, radio frequency power flows through the wolluton wire 84. the heating of which by the radio frequency currents changes its resistance them by unbalancing the bridge 43. and giving an indicating deflection on the meter 44 which is approximately proportional to the radio frequency power.

The power at the output cable I! of the sig nsl generator is calibrated by means of the precision bolometer 42 matched to the output cable-for s specified indication (normally midposition) on the output meter 44. when the oscillator plate voltage is manually adjusted to give the standard indication on the output meter 44, the available voltage at the output terminal can be read directly from suitable attenuator calibration curves (not shown). Due to the use of (a) a non-resonant bolometer 42 for monitoring the input to the attenuator on; to) small diameter of the attenuator 30 having low frequency correction; and (c) the lossy cable It for determining internal impedance oi the sig- 'nal generator, the voltage out ut of the signal generator for standard setting of the monitor 42 and a given setting of the attenuator Is varies but little with frequency.

In Fig. 2 of the drawings the oscillator In and bolmieter 42 of the Fig. 1 signal generator are shown in connection with a pulser so and a power supply circuit MI. The power supply circuit IN is of the voltage-regulated power supply ty e employing a full-wave high vacuum rectifier tube 380 and an no filter 351 of the condenser-input type. The output of the filter -"I through a dropping resistor 332 to two voltage regulator tubes "3 and "4 connected in series. The lead H0 is connected between the voltage regulator tubes 353 and 354 to provide the plate voltage for the oscillator iii. A voltage regulator tube 6|, connected to function as a cathode follower, supplies the direct current for the bridge circuit 43 in accordance with adjustments of a otentiometer 60.

The signal generator is designed to give either continuous unmodulated or modulated continuous wave or pulsed output of known amplitude and duration. The continuous wave output is calibrated by means of an external bolometer (not shown). The pulser 50 is designed 'so that potentials of all electrodes of the oscillator tube is during operation to produce pulses assume values identical with those obtained during C. W. operation. Thus, the power output duringthe pulse is the same as for (C. W.) condition. This is checked by observing the amplitude of the radio frequency pulse on the cathode ray tube of the receiver being tested. The fact that the amplitude of the radio frequency pulse does not change with pulse length is a direct indication that even the shortest pulse of the radio frequency amplitude reaches its steady continuous wave value.

The function of the pulser 60 is to bias the oathode II of the tube ll of the oscillator i so that the tube is cut of! during the period between pulses, and to let the cathode II assume its normal bias for the duration of the pulse. This system of pulsing has the advantage that short rectangular pulses, of the order of one microsecond, can be easily obtained. The only re, quirement on the pulser is the ability to generate short pulses with "steep" sides. Variations of pulse amplitude and irregularities of the pulse near the top have no effect on the envelope of the final radio frequency pulse. The length of the pulse is controlled solely by the constants of an artificial transmission line 01, presently to be described, and thus is also independent of variation in pulser tube characteristics or other factors.

The pulser produces pulses adjustable between 2 and 80 microseconds duration at a pulse repetition rate variable from 80 to 2500 pulses per second. Provision is made for applying external synchronizing voltage in which case the pulse repetition rate can be extended to 4000 pulses per second. The pulse delay circuit allows delaying the final pulse relative to the external synchronizing pulse by an interval of from 8 to 300 microseconds.

In particular, the pulser 50 comprises a 6SN7 tube 65 in an asymmetrical multivibrator circuit, the output of which keys an 884 tube 06. The open-circuited artificial transmission line 81 is included in the cathode circuit of the tube 60 and controls the length of time during which tre tube 68 conducts. The negative pulse voltage produced across plate resistor 08 of tube 08 is applied to the grid of a 8Y6 tube 69, so that the current which normally passes through the tube 60 is cut of! during the pulse. The normal current of the tube 69 fiows through a resistor 10 (270 ohms) common to the cathodes of the tube 08 and the oscillator tube ii and is sufilcient to cut off the current in the oscillator l0. By interrupting current in the tube 09 the negative pulse'restores normal operating bias on the oscillator l 0 for the duration of the pulse. An external triggering signal from a suitable source can be supplied-at a terminal H to synchronize and stabilize the multivibrator 05. The synchronizing input can be sinusoidal or other wave shape.

Considerable attention in the design of this signal generator has been given to radio frequency shielding and filtering in order to minimize leakage and radiation from the generator at the very high frequencies generated. The'oscillator circuit, the wavemeter circuit, and the attenuator input circuit are all of cavity type so that radio frequency fields are confined within separate and associated metal enclosures. All leads carrying direct current or pulse currents pass through radio frequency filters built within this primary shielding. The complete oscillator-attenuatorwavemeter assembly is placed inside a metal housing or compartment (not shown) so constructed and arranged as to constitute a second shield. As shown in Fig. 1 all leads from this compartment are brought out through a specially designed filter circuit 15. The filter circuit is con-' structed in accordance with the principles disclosed in the application of Andrew V. Haeff, Serial No. 635,121, filed November 29, 1945, for Radio Frequency Filter. The filter 15 consists of six isolated compartments (not shown) for the six separate leads thereof. Each compartment 8 houses a filter unit of the construction shown in Figs. 4 and 5 formed of a. two-section constant- K type filter assembly made up of three capacitors l0, l1, and 18 connected by two chokes l8 and 80 each of No. 34 wire, one-eighth inch diameter and two centimeters long.

Five of these filter units have a capacitance of 200 mmfd., while the sixth, through which the pulse is applied to the cathode of the oscillator, has a total capacitance of the order of 75 mmld. in order to pass narrow pulses.

As previously mentioned. one of the important uses of this pulses signal generator is in connection with the alignment, testing, and measurement of overall sensitivity of ultra-high frequency receivers such as, for example, of the type employed in radio-echo ob ect detecting and ranging equipment, and in the measurement of gain. noise factor, response to pulse, overload. and other characteristics of the receiver. For the measurement of sensitivity, for example, a pulse signal is fed to the rceiver to be tested, and observations are made on the cathode ray tube normally associated with the indicator unit used with such equipment. The pulse voltage corresponding to the minimum signal discernible on the cathode ray indicator tube is taken as a measure the overall signai-to-noise ratio of the receiver. In making these measurements on complete receiver-indicator units of radio-echo ob ,ect locating equipment, it is convenient to have the signal generator pulser triggered by the pulse which is used for triggering the indicator unit and, also, to delay the artificial echo pulse by an adjustable amount in order to observe the pulse at a convenient position on the screen of the cathode ray indicator tube.

The pulser, which is to be presently described, has been designed to fulfill these requirements, and it has been incorporated in the signal generator circuit of Fig. 3.

Referring to the pulser circuit shown in Fig. 3 tube It, a type 6J5 triode, is used only when it is desirable to synchronize the pulser from an external source. The switch I32 (D. P. D. T.) should be set at "plus" or "minus" position to agree with the polarity of the external synchronizing pulses to be used (waveform not critical). Thus, with negative external pulses and switch I32 set at "minus" position, positive pulses are delivered to the grid of tube IOI which is a type 884 gas-filled triode used as a relaxation oscillator. Resistors I05 and I00 constitute a voltage divider, and capacitor I01 (12 mid.) holds the plate of tube I03 at constant voltage. When tube I08 fires, the sudden fiow of current through resistors I08 and I00 causes a sharp rise of the cathode potential which cuts of! the tube. The cathode voltage then starts to leak of! to ground through resistors I08 and I00 and thus there is built up a potential difference across capacitor IN. This continues until the firing point is reached, when tube 303 again fires, discharging capacLor H0 and causing a sharp rise in cathode potential.

The charging rate of capacitor H0, and consequently the time required for each cycle, is variable by means of variable resistor I09, which is the pulse-rate control. Although the setting of resistor I00 governs the free-running rate of the oscillator to 2500 cycles per second), it readily synchronizes with an external pulse rate within this range, applied through tube IM to the grid of tube IOI. An external pulse reduces the negative bias on the grid of tube I08 and causes the 1b tube to fire without waiting for the cathode voltage to drop to the firing point necessary for freerunning oscillation. Waveform A (Fig. 6) shows the shape of voltage pulse at the cathode of tube 303 with respect to ground.

The pulse at the cathode of tube 303 is applied through capacitor 3 to the grid or tube SIZA, which is the first section of a type 6SN7 dualsection triode. Because of the low capacitance (200 mmi'd.) of capacitor II I, it charges quickly and the pulse reaches the grid of tube in as a sharp ositive pip (waveform B of Fig. 8). This positive pip starts plate current flowing in tube 3I2A which is normally biased below cutoil'. The fall of plate potential resulting from this current is applied through capacitor II! to the grid of tube driving it to cutoif. The charge of capacitor 3|! leaks oil through resistor I and lit and allows plate current to new again in the second section (tube IIIB) of the pulse delay multivibrator 3I2. Tube IIIA in the meantime has quickly biased itself below cutofl again. During the time the plate current of tube 2B is cutoff there is a rise in plate voltage, and a ositive voltage pulse of the form shown by waveform C, Fig. 6, is obtained. The length of this pulse is variable with the pulse-delay control (resistor 3 l5) A differentiating circuit is employed in the output of tube 3MB consisting of capacitor tit and resistor 3I1 with associated resistors II8 and 3I9 in the grid circuit of tube 320. The result is a sharp ositive pip from the leading edge of the output pulse from tube NIB and a sharp negative pip from the trailing edge, which are applied to the gr d of tube 020 (see waveform D, Fig. 6).

Tube 320 is a tvpe BAG? tube used, in coniunction with tube 32I, as a keyer-multivibrator and is normally cond cting. The positive pip applied to its grid has little effect on the plate current of tube 320 because it is already high, but the negative pip cuts down the plate current and st rts the act on in the multivibrator circuit of which tube 320 is a part. Thus by varying the length of the ulse from tube "2B, the spacing between the positive and negative pipe is varied.

This spacing is the time delay between the start of a synchronizing pulse applied to tube "I and the start of the output pulse.

The rise in plate potential, resulting when the plate cm-rent of tube 320 is cut down by the negative pip from tube "2B, is fed through capacitor 322 to the grid of tube "I which is a type 6AC7 operated as a multivibrator in con- Junction with tube 320. Tube III is normally cut off, and the positive pulse from tube 320 starts plate current flowing in tube I2 I. The plate po tential drops and this drop is fed back to the grid of tube 320 through capacitor 020. This process is regenerative and continues until cutoff occurs in tube 320 and saturation current flows in tube 32L The charge on capacitor 020 then leaks oil through resistor 3|! and associated resistors lie and us until tube #20 again conducts and tube '32! is cut on. Waveform E, Fig. 6, shows the pulse form at the plate of tube "0.

The pulse length which corresponds to the time tube 320 is cut of! is adiustable by means of resistor til, the pulse-length control.

The cathode resistor 320 of tube 320 is also the cathode resistor oi the oscillator tube II. With tube 320 conducting. its plate and screen currents cause sumcient drop across resistor I" to cut off the radio frequency oscillator tube I l, the grid of which is grounded. when tube 820 "10 oscillates. A switch 32! mounted on the pulserate control (resistor 309) is provided to disconmet the pulser circuit when continuous-wave (C. W.) operation is desired. Waveform 1', Fig. 6, shows the pulse forms at the cathode of tube 320 with switch 321 closed. With no external synchronizing voltage applied to the pulsar circuit the frequency can be varied from about 200 cycles per second to about 1500 cycles per secend. with the external synchronization. the range is from about 100 cycles per second to about 2500 cycles per second.

The oscillator of Fig. 3 includes the tube II and cathode and plate cavities coupled thereto. The cavities are formed by three hollow cylinden; or tubular members IOI. I02 and I03 arranged concentrically one inside the other. The middle or grid cylinder I02 is provided with an electrical connector (not shown) having spring lingers arranged in a ring which make contact with the disk-like grid terminal of the "lighthouse" oscillator tube II, and the outer or cathode cylinder IN is provided with a similar ring connector (not shown) which makes similar contact with the cathode terminal band connected to the cathode It of tube II. The oath ode I! of tube II is insulated inside of the tube from the heater I05 by a mica washer (not shown). The cathode I3 is connected to the resistor 82! through the filter I5, and plate voltage is applied to oscillator tube II by means of line 0 and a filter is provided in the circuit of the heater I00 in a manner similar to the Fig. 1 arrangement.

Oscillations are maintained by an adjustable coupling capacitor 323, in the form of a 4-42 screw, which provides feed-back coupling be tween the cathode and plate cavities. The screw 32: is supported in the cathode cylinder I II and extends through a clearance hole in the grid cylinder I02. The capacitance of screw 323 is varied by varying the distance of its inner end from the inner or plate cylinder I03.

As mentioned heretofore. in order to make tracking possible a compensating impedance loop 30 is connected between the plate and grid cylinders I02 and i0! close to the tube II. The loop loads the plate-grid cavity and tracking can be accomplished only when the impedance of the loop and the impedance of the attenuator stub ILA-42B are equal over the whole frequency band.

In order to obtain uniform output of the signal generator over the desired frequency range the "stub" line 3i energizing the attenuator 30 is made in two sections "A and 30B of diiierent length having different characteristic impedancee. For proper tracking the compensating impedance loop 80 is also made in two sections having lengths and impedances corresponding to those of the stub line "A and 3213. In addition, undesirable second harmonic output is suppressed by loading the stub line eitherby properly positioning the bolometer element 34 and/or by adding special loading resistors along the stub line. The optimum position of the harmonic suppressing resistor is usually a distance approximately'one electrical quarter wavelength (at the harmonic frequency) away from the shorted end of the stub line. This results in maximum load in: of the stub line at the harmonic frequency while exhibiting only moderate loading effect at the fundamental frequency. This arrangement results in uniform power output over the freis cut off this extra bias is removed and tube II 70 quency range and minimizes the amount of harmonic voltages appearing at the output terminal of the signal generator.

The power supply circuit for the Fig. 3 signal generator is similar to the power supply 20l described heretofore and includes the full wave rectifier tube 350, filter 35l, dropping resistor 352 and voltage regulator tubes 353 and 354. Power for the pulser and the bridge circuit 43 are derived directly from the output of the voltage regulator tubes 353 and 354 and plate voltage for the oscillator i is supplied through voltareontroller tube 355.

Tube 355 is a type 6AC'7 tube with s reen and suppressor gr'ds tied to the plate acting as a cathode follower. Varying the grid bias on the control grid, by means of potentiometer 351. changes the p ate resistance of the tube and gives an adiustable voltage bet een cathode and ground. wh ch is applied to the plate of oscillator tube II. The foregoing arrangement insures th t the plate vo tage of the oscillator remains substantiall constant under pulse or continuous wave operation and yet may be re dily ad- .iustable. Res stor 358 provides cathode bias for vol a e-controlled tube 355. Switch 360 (S. P. S. T.) controls the application of plate vo tage to osc ator tube H and makes it possible for all elements of tube H to heat up to operati tem erature before plate voltage is ap ed thereto.

When e ternal modulation of the si nal generator out ut is des red. the modulating signal is applied between the "Ext. Mod." and "Gnd terminals which are indicated at the extreme left of F 3. This signal is carried through capacitor "3 to the control grid of voltage-controller tube 355, and thus modulates the plate-supply voltage and output of oscillator tube II.

It should be distinctly understood that the specific arrangements of the circuit elements of Figs. 1. 2 and 3 are illustrative but not limitative of the present invention, and that the values of the circuit elements may vary and change when used for a specific range of ultra high frequencies. Thus. the signal generator shown in Figs. 1. 2 is des gned primarily for use in the ultra high frequency range sub tantially between 2000 and 4000 megacycles while the sytsem illustrated in Fig. 3 is de igned for use in the frequency range of from about 500 to 1300 megacycles. the principles of the invention being applicable to both embodimentsshown.

Although several embodiments of the present invention have been di closed and described in detail herein it is to be expres ly understood that various chan es and substitutions may be made therein without departing from the spirit of the invention as well understood with those skilled in the art. Reference, therefore. will be had to the appended claims for a definition of the limits of the invention.

The invention described herein may be manufactured and used by or for the Government of the United states of America for governmental purposes without the payment of an royalties thereon or therefor.

What is claimed is:

1. In combination. an oscillator of radio frequency oscillations including a vacuum tube having anode. cathode and control electrodes, and an oscillatory circuit comprising a pair of independently tunable resonant cavities: an adiustable attenuator system; means coupling said attenuator system to one of said cavities; said coupling means including means providing a transfer of energy from said one of said cavities to said attenuator system at a point of low voltage; and a bolometer element capacitively connected across the R. F. oscillation circuit near the input to said attenuator system, said bolometer' element being connected to and forming one'arm of a four-arm balanced bridge circuit.

2. A system in accordance with claim 1 char acterized in this that filtering means are included in anode electrode circuit of said oscillator, the cathode electrode circuit of said oscillator and the bolometer input line to the bridge circuit.

3. A system in accordance with claim 1 characterized in'this that filtering means are included in the anode electrode circuit of said oscillator, the cathode electrode circuit of said oscillator, and the bolometer input line to the bridge circuit. said filtering means in the separate leads each comprising a two-section low-pass constant-K type filter assembly.

4. A signal generator comprising a radio frequency vacuum tube oscillator tunable over a band of frequencies, said oscillator having a pair of tuned circuits for determining the frequency of thesignals generated by said oscillator, said circuits comnr sing two independently tunable resonant cavities: means providing feedback between said cavities: a tunable attenuator system; means cou ling said attenuator system to one of said cavities: said cou ling means including means providing a transfer of energy from said one of said cavities to said attenuator system at a point of low-voltage: a cavity-type wavemeter loosely cou led to the other of said cavities, said wavemeter includin a rectifier and an indicating device in the rectified output of the wavemeter circuit for indicating resonance thereof; a bolometer element capacitively connected across the R. F. oscillator circuit near the input to said attenuator system. said bolometer element being connected to and forming one arm or branch of a Wheatstone bridge circuit the remaining three branches of which contain ohmic resistances. said bridge including an indicating meter; and means for supplying triggering signals to said oscillator from a source of pulse signals.

5. A signal generator comprising a radiofrequency vacuum tube oscillator circuit of the grounded-grid type tunable over a band of frequencies, said oscillator having a pair of tuned circuits for determining the frequency of the signals generated by said oscillator. said circuits com rising two inde endently tunable resonant cavities: means providing feedback between the two resonant cavities: a tunable attenuator; means coupling said attenuator system to one of said cavities: said coupling means including means providing a transfer of energy from said one of said cavities to said attenuator system at a point of low voltage: a cavity-type wavemeter loosely cou led to the other of said resonant cavities. said wavemeter including a rectifier and a meter in the rectified output of the wavemeter circuit for indicating resonance thereof: a bolometer element capacitively connected across the R. F. oscillator circuit near the input to said attenuator system, said bolometer element being connected to and forming one arm or branch of a Wheatstone bridge circuit the remaining three branches of which contain ohmic resistance, said bridge including an indicating meter: and a pulser system compris- 338 vacuum tube asymmetrical multivibratcr circuit, a pulser tube having an anode, a cathode, and a control grid. said pulser tube being keyed by the output of said multivibrator, an artificial transmi sion line in the cathode circuit of said pulser tube, and a keyer tube.

6. A signal generator in accordance with claim 5, including filter circuit elements connected to the anode electrode circuit of said oscillator, he heater circuit oi said oscillator, the cathode electrode circuit of said oscillator, the bolometer input line to the bridge circuit, and the line connecting the rectified output of said wavemeter to the indicating meter therein.

I. A signal generator comprising a radio-frequency oscillator system of the grounded-gnd type tunable over a band of frequencies and including a high frequency vacuum tube having cathode, anode and control el ctrodes, said oscillator system having a pair 01 tuned circuits for determining the frequency of the signals generated by said oscillator, said circuits comprising two resonant cavities, one in the grid-plate circuit and the other in the grid-cathode circuit oi said oscillator; means for independently tuning said cavities; means providing feedback between the two caviti s, a wave-guide t e attenuator system; means coupling said attenuator system tothe cathode cavity of said oscillator: said cou ling means including means providlng a transfer of energy from said one of said cavities to said attenuator system at a point of low voltage; a cavity-type wavemeter including a rectifier, a tunable ouarter-wave coaxial line section loosely cou led to the plate cavity of said o cillator, and a meter in the rectified output of the wave eter circuit for indicating resonance of the wavemeter; a bolometer element capacitlvely connected across the R. F. oscillator circuit near the in ut to said attenuator svstem, said bolometer element being connected to and iformina one arm or branch of a Whe t tone brid e circuit the remaining three branches of which contain oh ic resi tances, said brid e includin an ind cat is meter: and a pulser svstem comprising a phase invert r circuit a relaxation oscillator ci cuit. a ul e-delay multivibrator circuit. and keyer-multivibrator circuit in the form of a v cuu tu e a mmetrical multivibrator the out ut of which kevs said oscillator tube.

8. A ignal generator in accordanc with claim 5 inc udin "cans a sociat d ith the cathode circuit of said oscillator for rendering said pulser system ineffective and for connection of said oscillator to an external source of modulating pulses.

9. A signal "op rator in accordanc with claim 4, includ ng shield ng means individu l with the o cillator circuit. the wavemeter circuit and the attenuator in ut circuit, respectively, each shielding means forming an int ral part of its a sociated circuit for preventing R. F. radiation lo s therefrom. and means constructed and arran' ed to form a shieldin housing enclosing the complete o cillator-attenuator-wavemeter system to provide a R. F. second shield therefor.

10. A hi h frequenc oscillation generator comprising, in combin tion, a h gh frequency vacuum tube having cathode. anode and control electrodes. and a pair of tunable oscillatory circuits coupling together certain of said electrodes and com rising a plurality of conc ntrically disposed hollow cylindrical conductors connected to ether at one end and arran ed to form gridplate and grid-cathode resonator cavities: a coaxial stub line shorted at one end and capacitiveiy coupled at the open end to said grid-cathode cavity, an attenuator system: said attenuator system including a loop inductively iupled to said coaxial stub line at a location near its shorted end; and a bolometer element connected to said stub line adjacent the shorted end thereoi, said bolometer element forming one arm of a balanced four-arm bridge circuit.

11. A high frequency oscillation generator comprising, in combination, a high frequency vacuum tube having cathode, anode and control electrodes, and a pair of tunable oscillatory circuits coupling together certain of said electrodes and comprising a plurality of concentrically disposed hollow cylindrical conductors connected together at one end and arranged to form gridpiate and grid-cathode resonator cavities; a coaxial stub line shorted at one end and capacitively coupled at the other end to said grid-cathode cavity; an ad ustable attenuator system including a loop inductively coupled to said coaxial stub line at a location near its shorted end: a bolometer element connected to said stub line adjacent the shorted end thereof,,said bolometer element forming one arm of a balanced four-arm bridge circuit, said stub line comprising two cylindrical sections having different lengths, diameters and impedances; and a compensating element connected between the plate and grid cylinders close to said oscillator tube, said compen ating element being constructed and arran ed to have electrical dimensions and impedances corresponding to those of said stub line for loading the grid-plate cavity whereby pro er tracking of the respective tuning elements of said cavities is obtained over the whole frequency band.

12. In co bination, an oscillator including a vacuum tube having an anode, a cathode and a plurality of cont ol electrodes and an o cillatorv circu t compri ing a pair of independently tuned re onant cavities, an adiustable attenuator sy tem, and means cou ling said attenuator svstem to one of said cavities. said coupling mean compris ng a stub line havin one end oapacitivelv cou led to said one of said cavities, means short-circuiting the other end of said line and means cou ling said attenuator system to said l n ad ac nt said other end.

13. In combination, a high frequency oscillator com ri in a vacuum tube having a cathode, an anode. and a control grid, a pair of concentricallv dis o ed hollow cvlindrical conductors connected to form grid-plate and grid-cathode resonance cav ties. gang tuning means for synchronou lv tun n" sai cavities, a tuneable line having on end connected to one of said cavities at a noint of hi 'h volta e and the other end thereof connected to a point of relatively low volta e. an atte uator system cou led to said line ad acent said other end, compen ating impedance means included in the other of said cavities end means tuning said line to match the imedance of said cavities.

14. In com ination, a radio frequency pulse generator includin' a continuous wave radio frecuency oscillator having an electron discharge device provided with a bia ing r si tor in the cathode circuit thereof, a periodically conductin electron tube having a cathode circuit including said biasing resistor for periodically blocking said device so that radio frequency pulses are produced by said o cillator, and means m asuring the radio frequency power of said pulses, the lest-named means includin radio frequency power measuring means coupled to the output of said oscillator and means operable 15 to disconnect said biasing resistor from the cath- Numb r ode circuit of said tube so that said measuring 2,098,051 means produces an indication of radio frequency 2,181,568 pulse power as a function of continuous wave 2,204,179 radio frequency power. 5 2,235,521 ANDREW V. HAEFF 2,255,727 'I'HURE E. HANLEY 2,266,401 CHARLES B SMITH. 2, 76,99 2,277,638 REFERENCES CITED 10 2395585 The following references are of record in the 2,302,369 file 01 this patent: 2,333,638 UNITED STATES PATENTS g'zggfgg Number Name Date 15 1,901,741 Fetsch Mar. 14, 1933 1,950,410 Lake Mar. 13, 1934 16 Name Date Lord Nov. 2, 1937 Kotowski Nov. 28, 1939 George June 11, 194 Higgins Mar. 18, 19-- Allison Sept. 9, 1941 Reeves Dec. 16, 1941 Milinowski Mar. 17, 1942 George Mar. 24, 1942 Lindquist Sept. 15, 1942 George Nov. 17, 1942 Shepard Nov. 9, 1943 Korman July 2, 1946 Wagner July 16, 1946 

